The present invention relates generally to the recovery of hydrocarbons by oil shale retorting and coal gasification, and more particularly to a retorting and combustion arrangement wherein oil shale is pneumatically displaced through a combined retorting and combustion system for effecting the sequential retorting and combustion thereof in an efficient manner.
In oil shale retorting operations, the heat of combustion of hydrocarbons in the oil shale has been used to provide the heat source necessary for effecting the "driving off" (retorting) of the hydrocarbons from the oil shale. The use of a retort with a moving bed, countercurrent solid/gas flow arrangement has been used for this purpose and has provided a substantial advancement in oil shale retorting operations as well in coal gasification processes. In the moving bed retort, the combustion of hydrocarbons in the oil shale takes place near the bottom of the moving bed with resulting hot gases rising through the overlying bed material into a zone where the oil shale retorting takes place. The close proximity between the retorting zone and the underlying combustion zone provides efficient heat transfer between the hot gases and the oil shale especially since this heat transfer takes place in a countercurrent flow mode. This particular heat transfer relationship is one of the main features of this type of moving bed retort.
While the aforementioned solid/gas, retorting-combustion system and other similar systems have achieved some success in oil shale retorting, there are several attendant problems or shortcomings which detract from the efficiency and overall success of these systems. As pointed out above, the combustion zone of a moving bed retort is near the bottom of the housing and as the hot combustion gases rise through the overlying packed column, the sensible heat from the combustion process retorts the oil shale by forcing the kerogen in the shale to decompose through a destructive process into recoverable gaseous hydrocarbons. The inherent variation within the packed bed or column of oil shale in this retorting process causes some nonuniform distribution of the hot combustion gases rising therethrough, particularly in large bed facilities where large quantities of solids are packed into a column in a countercurrent, heat-exchange relationship with the gaseous products of combustion. In such facilities, whenever an obstruction to the gaseous flow through the overlying solids occurs, the rising gases form channels through the solids in areas having the least resistance to flow. When such channeling occurs, an excessive portion of the oil shale will not be adequately retorted since the temperature distribution through the oil shale bed is no longer sufficiently uniform. Thus, oil shale retorting becomes somewhat inefficient by using moving bed retort techniques, especially on a relatively large scale as would be used in commercial applications.